1. Field of the Invention
The present invention is directed toward a methodology for independent longitudinal wheel slip and lateral skid control utilizing a traction control and a stability control system in cooperation with a drive torque distribution control system capable of individual wheel drive torque modulation.
2. Description of Related Art
A vehicle system may consist of a variety of control actuators (brakes, drive torque distribution, suspension stroke control, tire load control, etc.) capable of modulating the forces at each wheel for the purpose of maintaining traction, achieving vehicle dynamics responses and ensuring vehicle yaw stability. Each actuator is generally driven by it's own, dedicated ECU (electronic control unit) to achieve a specific function. Some actuator ECU's may share status information in order to understand each ECU's operational state, or failure condition.
In the area of traction control leading to stable vehicle response, vehicle stability assist systems or traction control systems usually are based on brake technology and command changes in total system drive torque through engine control (changes in engine torque) and modulation at each individual wheel by brake torque application. In this manner it is possible for the Traction Control System (TCS) or Vehicle Stability Assist (VSA) controller to limit the overall wheel slip and/or lateral skid and to achieve a stable vehicle response.
However, since the control of the total drive torque, as modulated by engine torque control, is not directly related to each wheel's drive torque condition (i.e. an engine torque change affects the overall torque but not specifically an individual wheel drive torque), it has limited effectiveness in terms of simultaneously keeping vehicle momentum and allowing a stable vehicle response while controlling wheel slip and/or lateral skid of an individual wheel.
The TCS and/or VSA controller is usually designed to operate in a slip-based and/or lateral skid-based feedback manner and is usually designed and implemented with high fidelity for this purpose. However, it does not, in the current state of the art, have the capability to modulate individual tractive torques at each wheel through changes in drive torque distribution of the driveline.
The TCS and/or VSA controller, in the current state of the art, has the capability to:                Calculate a reference vehicle speed;        Calculate each wheel's traction condition (either longitudinal slip rate or lateral skid);        Compare to a threshold speed and/or longitudinal slip and/or lateral skid;        Judge the proximity to a potentially unstable vehicle motion; and,        Calculate a required application torque (usually brake) to control wheel spin and/or wheel lateral skid and/or vehicle motion.        
It has been found that the conventional TCS or VSA operation has limited ability to respond to operation on low coefficient surfaces during turning and hard acceleration. During vehicle launch on low coefficient surfaces and while turning in particular, there is a higher likelihood of loss of traction and potentially ensuing loss of yaw stability and/or vehicle maneuverability.
In these types of events, the current state of the art in traction control systems reduces engine torque in addition to applying brakes to the slipping and/or laterally skidding wheel(s) to control the vehicle motions, wheel slips and lateral skids. This usually results in a hesitation to the overall vehicle momentum because the amount of engine torque reduction required to reduce the most critical wheel slip and/or lateral skid also reduces the amount of tractive effort on other wheels that may not be in an excessive slip and/or lateral skid situation and may even have additional traction capability. This hesitation may, in some cases, be detrimental and could lead to problems in some situations, such as hill climbing or operation in tightly confined spaces where acceleration is requested but not delivered due to this hesitation.
Therefore, there exists a need in the art for an improved traction control type system, wherein reduction in engine torque is prevented, if possible, or delayed, and wherein drive torque provided to a slipping and/or laterally skidding wheel is reduced, so as to better match the traction conditions being experienced by the vehicle, thereby providing better handling, especially during low speed, tight turn acceleration on low coefficient of friction surfaces.